1. Field of the Invention
The invention pertains to the field of signal detection and more particularly to the detection of signals having harmonically related components.
2. Description of the Prior Art
The physical processes involved in the emission, reflection, and transmission of electromagnetic or acoustical signals often produce secondary signals which are harmonically related to the primary signal. Emissions as for example, from non-linear waveform transformations, introduce harmonical components that are phase coherent with a fundamental frequency. An illustration of this is the reflection of electromagnetic signals from the boundary of two dissimilar metals. The dissimilarity of the metals forms a diode junction and an incident signal is subjected to the non-linear diode response before it is reflected-from the boundary. Another source of electromagnetic harmonic signal generation is the reflection from aircraft propeller and jet engines. Propeller and jet engine rotation induce a complex modulated pattern on reflective signal that is rich in harmonics. Since this modulation varies with the engine and aircraft configuration a spectral analysis of the modulation signal may be used for aircraft identification.
In the prior art these complex signals are filtered for harmonic separation, envelope detected, each detected signal integrated, and the resulting integrations summed. This process provides improved detection over systems that operate with the primary signal only when the receive signal is sufficiently stable to maintain the signal components within the designated filters. The signal frequency in many applications is not stable and the generated signal components may wander out of the designated filters. Compensation for this signal frequency instability may be accomplished by providing additional filters of sufficient number and bandwidth to span the frequency space between basic harmonic filters. The output of these filters may then be enveloped detected and subsequently used for spectral display or post detection processing. Since no stable reference exists a sum of integrated signals for the deflected components can not be realized and the output of each filter must be individually displayed and processed.
In accordance with the present invention a system for detecting signals having harmonically related components employs a plurality of harmonically related filter banks, the first bank having filters of equal bandwidth and center frequencies equal to the fundamental of the received signal plus an integer multiple of the common bandwidth such that the center frequency separation is equal to the common bandwidth. Subsequent filter banks, as for example that for the kTHharmonic, have filters with equal bandwidths that are k times the bandwidth of the filters in the fundamental frequency filter bank and have center frequencies that are k times the center frequencies of the fundamental filter bank, thus providing center frequency separations equal to k times the common bandwidth of the fundamental filter bank. In this manner each of the filter banks, succeeding the fundamental filter bank, have filters correspondingly related to the plurality of filters in the fundamental bank. These corresponding filters are summed in an escalating manner (fundamental plus second harmonic, fundamental plus second, and third harmonics, etc.) to form groups of a multiplicity equal to the number of filters in each filter bank. The sums of each group are coupled to a processor wherein a dynamic integration or other suitable processing is performed to establish a signal detection and harmonic content.
In a second embodiment of the invention, employed with ten signals within a finite band that extends from d.c., two filter banks are employed. The first filter bank contains a plurality of filters, each of equal bandwidth with center frequencies commencing at and separated by the bandwidth of the filters. The output terminals of these filters, which may number Q, are grouped such that, for example, the first M output terminals are coupled to amplifiers and given equal predetermined weights. A second group, commencing with the second output terminal to the output terminal 3M+1 are coupled to amplifiers and given a second predetermined weight. Since this group contains the output of three times as many filters as the previous group, the amplifier outputs are combined in threes to provide ultimate output terminals of a multiplicity equal to that of the first group. Each filter group formed has the first filter output terminal of the group being the filter output terminal that is the second filter output terminal of the previous filter group and has the last filter output terminal that which provides 2M more output terminals to the group over the number of output terminals utilized in the previous group. The signals at the group output terminals are each given a weight for that group and the output terminals of the weighting amplifiers are combined to provide ultimate output terminals of a multiplicity that is equal to the multiplicity of the ultimate output terminals of all the previous filter output terminal groupings.
The second filter bank possesses a multiplicity of filters having bandwidths that are substantially equal to the bandwidth of the filters in the first filter bank and center frequencies that are upwardly displaced from the center frequency of the corresponding filter in the first filter bank by one-half a bandwidth. The output terminals of this second filter bank are grouped as described above for the first filter bank except that the first grouping comprises the first 2M output terminals. The totality of ultimate output terminals from the two filter banks form K ordered groups with equal numbers of ultimate output terminals therewithin. The ultimate output terminals of each group are correspondingly coupled to summation networks and the output terminals thereof are coupled to processors in like manner as the above discussed embodiment.